The principles of acoustic levitation are well established. It is known that sound exerts a small but continuous force on materials in a sound field over and above the pressure oscillation occuring at that frequency. Various schemes have been devised whereby such continuous forces are maximized sufficiently to enable small objects to be suspended in a sound field without visible means of support.
The potential applications for acoustic levitation, positioning and manipulation are numerous and varied. Many potential applications exist whenever there is a need to hold, move, store or position an object without contact with any surface, particularly if such contact would contaminate or damage the object or otherwise interfere with some desired property or state of the object. For example, an object that is melted by conventional means at high temperatures will be contaminated by the container, and acoustic levitation offers the possibility of containerless melting as well as other containerless or non-contact processing involving, for example, chemical reaction, alteration of physical shape, coating, combining, conveying, and the like.
Acoustic levitation also lends itself to manufacturing processes in outer space by preventing drift of the materials being processed. In this connection, several proposals have already been made for the processing of acoustically positioned objects in future space stations.
Of the acoustic levitation systems heretofore proposed, all have exhibited serious drawbacks or limitations that affect practical usage or otherwise restrict performance under a variety of conditions. All previous systems utilize resonant cavities that must be carefully tuned and because of the cavity nature are limited in physical geometries. For example, U.S. Pat. No. 3,882,732 describes a resonant rectangular chamber utilizing three sound transducers arranged in three normal axes. The system is used to establish a standing wave pattern to urge an object toward a zone of minimum pressure. Tuned cylindrical chambers have also been proposed but similarly suffer from the requirement to be tuned.
In another proposed system, a single sound source is used, and a large reflector is placed at a critical distance from the sound source to produce standing waves.
In all of the foregoing, the geometry of the system is critical and variations in temperature cause the system to detune. If a reflective surface is used to produce a standing wave, the distance between the sound source and reflective surface must be maintained at n (x/2), where x is the wavelength of the sound, and n is a whole number. If the temperature should change, the wavelength of the sound is also changed. As a result, the system will no longer be in resonance, the standing wave will be destroyed, and the levitation will be lost.